Gas generating compositions have long been used for a multitude of purposes. The requirement for vehicular, especially automotive, airbags in passive restraint systems designed to protect drivers and passengers in the event of a collision has produced a substantial amount of research for the ideal gas generating composition for this purpose. The ideal gas generating composition should be a thermally stable, cool burning, noncorrosive composition that generates a large volume of substantially ash or solids-free clean, nontoxic gas. The ideal gas generating composition should also be storage stable so that it ignites effectively and burns efficiently when needed. While the prior art has proposed gas generating compositions that approach this ideal, it has not yet been achieved for automatically inflated structures such as vehicle airbags.
The current state of the art gas generating compositions, referred to in the gas generator technology as "propellants", typically include an ammonium nitrate oxidizer, either combined with a rubbery binder or in a pressed charge to form a pellet, which is stored until ignited to inflate the airbag or other structure. Various chemical additives, for example highly oxygenated fuels such as guanidine nitrate, aminoguanidine nitrate and oxamide are combined with the ammonium nitrate to aid ignition, modify burning rates, promote smooth burning and produce acceptably low flame temperatures. Combustion catalysts may be included in the composition to increase burning rate, promote ignition and low pressure combustion. However, the metallic additives often used in combustion catalysts produce solids in the effluent gas that may interfere with the gas toxicity, exhaust particulates, or inflation of the airbag or other inflatable structure.
Ammonium nitrate is the most commonly used oxidizer in these types of gas generator compositions. It is readily available, safe to handle, and inexpensive. Moreover, ammonium nitrate burns at low flame temperatures and burning rates to produce a nontoxic, noncorrosive exhaust. Primary disadvantages of using ammonium nitrate as the oxidizer in a gas generator composition are inherently low burning rates, higher pressure exponents, poor combustion at low pressures, and its tendency to undergo phase changes during temperature variations, which causes cracks and voids in the pellet. Cracked pellets are not likely to yield a reliable gas generator when needed. Crack formation can be minimized by employing a binder that is sufficiently strong and flexible to hold the composition together. Pellets formed without a binder additive will crack unless phase change additives are used and/or specific additional components or processing steps are employed.
U.S. Pat. No. 5,545,272 to Poole et al. is illustrative of an ammonium nitrate based gas generating composition for an automobile airbag. The mechanical mixture of ammonium nitrate, nitroguanidine, and a potassium salt described by Poole et al. suffers from some of the drawbacks discussed above, however. This type of composition is subject to the aforementioned ammonium nitrate phase changes due to temperature cycling. Since the composition does not include a binder or phase change modifying component and is not produced to modulate ammonium nitrate phase changes, cracks and voids in the gas generating pellet are a likely result.
U.S. Pat. No. 5,551,725 to Ludwig discloses an inflator composition for a vehicle airbag that includes an oxidizer, such as ammonium nitrate, and a fuel, which may be a nitro-organic, such as guanidine nitrate. The Ludwig composition, like the Poole et al. composition, would not avoid potentially detrimental ammonium nitrate phase changes.
In the foregoing gas generating compositions, as well as in other available gas generators, the burning rates tend to be low and the pressure exponent values tend to be high so that they are not as efficient as desired. These ballistic properties pose challenges in the design of a vehicle airbag unit. Low burning rates lead to high operating pressures and/or thin web designs. High exponents at low pressures lead to poor and variable combustion and unburned residues. Moreover, under these conditions, the thin web designs typically used for the gas generator charges weaken and become friable and are susceptible to vibrational damage so that the storage stability of the gas generator is compromised.
A need exists, therefore, for a thermally stable, storage stable gas generating composition characterized by both an increased burning rate and a lower pressure exponent than heretofore achieved that produces a clean, nontoxic, substantially ash- and solids-free gas at optimum combustion efficiency and operating pressure.